The line routes.IgnoreRoute("{resource}.axd/{*pathInfo}"); will prevent users to access .axd files which are present in the solution.Internally HttpHandler will handle that request so what if again user enters the file name in the browser with .axd extensions? so to prevent user doing that we need to specify that IgnoreRoute.Not only that .axd you can even stop any extensions from user to avoid accessing,, you can even prevent one of the controller methods like below